Penning gauge circuit improvement

ABSTRACT

An improved Penning gauge circuit in which the sensitivity and linearity of the response is maintained at pressures in the 10 4 and 10 5 torr range by using a device having nonlinear resistance in series with the sensing head.

United States Patent [54] PENNING GAUGE CIRCUIT IMPROVEMENT 1 Claim, 3Drawing Figs.

US. Cl 324/33 (Win 27/00 [50] Field oi Search 324/33, I32; 307/3 1 7,263

POWER SUPPLY METER ClRCUlT [ill 3,622,870

[56] References Cited UNITED STATES PATENTS 2,806,154 9/1957Steinbuch..... 307/263 3,379,967 4/1968 Herrwerth 324/33 PrimaryExaminer-Michael J. Lynch A!torneys Plante, Hartz, Smith & Thompson andRaymond J. Eifler ABSTRACT: An improved Penning gauge circuit in whichthe sensitivity and linearity of the response is maintained at pressuresin the i0" and 10- torr range by using a device having nonlinearresistance in series with the sensing head.

PAIENTEDNI1V23|9H 3, 622. 870

A A 2000 B Q C 3,

D 5 200 a: a: D U 3 3 20- 4 o PRESSURE (TORR) FlGURE 3 O lw POWER MSUPPLY 20 METER URCUIT vn FIGURE 2 POWER NoN-uNEAR ssusme READ-0uTSUPPLY CIRCUIT MEANS HEAD DEVICE HGURE I ALLEN R HAMILTON THOMAS R.METZGER INVENTORS BY L 1 PENNING GAUGE CIRCUIT IMPROVEMENT BACKGROUND OFINVENTION When measuring pressures over a broad range, e.g. I torr tol0'"-" torr in an enclosure that is being evacuated, different gaugesare used in combination to obtain accurate measurements. A gauge thatmeasures pressures accurately down to the 10' torr range will be used incombination with a gauge that measures pressure accurately from the 10"torr range to lower pressures. In this way, as the pressure drops belowthe measurement capabilites of one gauge, it is measured by another.Matching the gauges to accomplish this object has always been a problembecause of the measurement capabilites of the gauges. For example, aPenning vacuum gauge which exhibits linear behavior for pressures belowIO' torr, exhibits nonlinear behavior, in the 10"" and torr ranges. Thislimits the usefulness of that gauge in that it leaves a gap between thelowest pressure reading of a thermal conductivity vacuum gauge (PiraniGauge, thermocouple, etc.), which measures pressures down to l0""' torr,and the highest linear pressure reading of the Penning gauge. Thenonlinearity of the Penning gauge at pressures above lO' torr resultsfrom the fact that at those pressures, the current through the sensinghead is a function of both the pressure within the head and the currentlimiting resistance in series with the sensing head. Presently, knownvacuum gauges have attempted to solve this problem by varying thecircuit voltage or varying (in steps) the current limiting resistor inseries with the sensing head. Neither of these methods is satisfactorybecause changing circuit constants (e.g. voltage and/or resistance)adversely affects the equilibrium of the pressure-current relationshipof the gauge.

SUMMARY OF THE INVENTION It is our invention to make use of a devicehaving nonlinear resistance characteristics in combination with aPenning vacuum gauge circuit to obtain a substantially linear responsefrom the gauge circuit when the sensing head is subjected to pressuresin the 10" and 10" torr ranges. By proper selection of the nonlineardevice, based upon its voltage-resistance characteristics, it ispossible to obtain from the gauge circuit a substantially linearresponse between pressure and sensing head current at pressures in theIO' and lO' torr ranges.

It is, therefore, an object of this invention to provide an improvedarrangement for a Penning vacuum gauge circuit that will have asubstantially linear output as the pressure being measured in thesensing head drops below 10' torr.

It is another object of this invention to provide a vacuum gauge circuitwith a nonlinear element which is chosen to compensate for the nonlinerresponse from a pressure sensing head at particular pressures so as toextend the linear operating range of the gauge circuit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of apreferred vacuum gauge circuit arrangement.

FIG. 2 is a preferred arrangement of a gauge circuit showing a varistorin series with a Penning Discharge Vacuum gauge.

FIG. 3 is a graphic representation of the pressure-currentcharacteristics of a Penning Gauge Type GPD 101 for differentresistances in a series with the sensing head.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to drawing andmore particularly, FIG. 1, which shows a block diagram of a preferredvacuum gauge circuit for accomplishing the object of our invention. Thepower supply 1 for the circuit is connected to the sensing head 5through the nonlinear circuit means 3. The nonlinear circuit means maybe either a solid-state device or an electron tube. The readout device 7connected to the sensing head 5, comprises means to amplify and/ortranslate the signal from the sensing head into units of electricalcurrent or units of pressure.

Referring now to FIG. 2 which shows a partial schematic of a preferredembodiment of our invention. The power supply I for the circuit may beany standard high voltage (l-Skv) DC power supply capable of supplying acurrent of at least 3 milliamperes. The high voltage lead from the powersupply I is connected in series to a varistor 13 which in turn isconnected in series to the anode of a Penning Vacuum Gauge head 15 (TypeGPD-001). The varistor I3 which also serves as a current limitingresistor, which is preferred for use with a type GPD-I01 sensing head,is manufactured by Carborundum Company and is a type BNR, rated at 2milliamperes at 840 volts, and 0.09 milliamperes at 300 volts. Althoughvaristor 13, type BNR is preferred, other types of varistors,individually or in combination with fixed resistors, may be useddepending upon the results desired. The meter circuit 17 which isconnected to the cathode of the gauge I5 comprises a current sensingreadout device which may be an ammeter calibrated in units of electricalcurrent or units of pressure. Where desirable, range switchingcomponents and/or an amplifier may be used in conjunction with themeter. The ammeter which is preferred in combination with a type GPD-lOlgauge head 15 has a range of 0 to 200 microamperes.

FIG. 3 is a graphic representation of the pressure-currentcharacteristics for our invention as compared with previous PenningDischarge Gauge circuits having fixed resistance valves. Curve A is thecharacteristic curve for the circuit shown in FIG. 2 with a varistor inthe circuit. Ideally, the varistor changes its resistance so that thecurrent through the sensing head 15 remains a linear function ofpressure for the circuit in the 10'' and 10" torr ranges. The varistor,therefore, is chosen for the circuit by its electrical characteristicswhich will result in the circuit having a substantially linear responseas compared to a circuit having a fixed resistance value which does nothave a linear response to pressures above l0' torr.

Curves B, C and D indicate the current response of the circuit shown inFIG. 2 to changes in pressure when a fixed resistance value of I, 2 and3 million ohms respectively is substituted for varistor l3. Curve E isthe characteristic curve of the circuit when varistor 13 is removed fromthe circuit and no resistor is placed between the power supply 1 and thesensing head 15.

OPERATION Referring now to FIGS. I and 2, the circuit operates asfollows. When the circuit is energized, a potential is applied to thesensing head 5, 15. As the enclosure (not shown) to which the sensinghead 5, I5 is attached is evacuated, the current flowing through thegauge sensing head 5, 15 decreases. However, with constant potentialapplied to the sensing head, the current would decrease more rapidlythan the pressure, through the 10'' and 10" torr pressure ranges. It isin these pressure ranges that the nonlinear circuit device of theinvention serves its purpose, since as the current decreases, theresistance of the nonlinear device increases in the correct amount so asto maintain a linear relationship between pressure and current. Forpressures below l0""" torr, the resistance of the sensing head is solarge in proportion to the resistance of the varistor 13 that changes inthe resistance of the varistor have little or no effect on the circuitcurrent. But for pressure above I0 torr, the varistor is chosen to havea resistance large enough to afiect the circuit current. This providesthe person monitoring the pressure in the evacuated enclosure (notshown) with linear response in the 10" and 10" torr range. In actualpractice, we have been able to extend the linear relationship (currentvs. pressure) to about 8Xl0' torr range for a Penning Discharge Vacuumgauge. With other voltages, magnetic field strengths, and sensing headgeometry, it is possible to extend the linear range to l0"'' torr andhigher.

While we have disclosed a preferred embodiment of our invention, it willbe apparent that variations in the specific details of constructionwhich have been illustrated and gauge head; current measuring meansconnected to measure the ionization current produced in said gauge head;and varistor connected in series with said gauge head, said varistorhaving electrical characteristics such that when said envelope isevacuated a substantially linear relationship between pressure andcurrent in said envelope is obtained in the l0 and 10 torr range.

t l l I! I UNITED STATES PATENT ()FFICE CERTIFICATE OF CORRECTION PatentNo- 3,622,870 Dated November 22 m7] Invento )Allen R. Hamilton. et al Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1,

line 5, "l0' should read 10 line 6, "l0' should read 10" line 9, "l0'should read 10' line 10, "lO' should read 10 line 17, "l0' bothoccurrences, should read 10" line 18, "l0' should read 10- line 21, "l0'should read 10" line 24, "l0' should read 10' line 40, "10' should read10" and "10' should read 10 line 44, "l0' should read 10' and "l0'should read 10 line 48, "lo' should read 10' Column 2,

line 31, "1o' should read 10' and "lo' should read 10 2 RM po'mso (1069)USCOMM-DC 00376-P69 n U 5 GOVERNMENT PRINTING OFFICE 9Q! 0-356'53 line36,

line 54,

line 59,

line 63,

line 66,

line 67,

line 69,

line 72,

(SEAL) Attest:

Patent No.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated Nogember23.4211.

Column 2,

should read read

read

read

read

read

read

Inven Allen R. Hamilton et a1 It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

"sx10' should read 8Xl0' "l0' should read 10" EDWARD M.FLETCHER,JR.

Column 4, line 8,

"l0' should read 10" and "l0- should read Signed and sealed this 31stday of October 1972.

ROBERT GOTTSCHALK IRM PC4050 (10-59) Attub'tiug Officer Commissioner ofPatents USCOMM-DC 008764 69 us. GOVERNMENT PRINYING ornc: is" 0 Jl5-334

1. A vacuum gauge comprising: a Penning type vacuum gauge head of thetype having an envelope adapted for connection to a vacuum system, ananode, and a cold cathode electrode enclosed by said envelope, a voltagesource connected across the electrodes of said gauge head; a currentmeasuring means connected to measure the ionization current produced insaid gauge head; and a varistor connected in series with said gaugehead, said varistor having electrical characteristics such that whensaid envelope is evacuated a substantially linear relationship betweenpressure and current in said envelope is obtained in the 10 4 and 10 5torr range.